Precipitation-hardened soft magnetic ferritic stainless steels

ABSTRACT

A precipitation-hardened soft magnetic ferritic stainless steel comprises C: not more than 0.2 mass %, Si: 0.01-3.0 mass %, Mn: not more than 0.5 mass %, S: not more than 0.3 mass %, Cr: 12.0-19.0 mass %, Ni: 1.0-4.0 mass % and Al: 0.2-4.0 mass % and further contains at least one of Ti: less than 0.5 mass % and Zr: less than 0.3 mass % and the remainder being inevitable impurities and Fe, and has substantially a microstructure of a ferrite phase after a solution treatment and an aging treatment.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a precipitation-hardened soft magneticferritic stainless steel, and more particularly to aprecipitation-hardened soft magnetic ferritic stainless steel having notonly excellent magnetic properties and corrosion resistance but also ahigh hardness, which contributes to an improvement of a durability suchas a wear resistance, a resistance to a buckling and the like in amovable part of a magnetic circuit apparatus, and a good coldworkability. Especially, it is preferable to use such a ferriticstainless steel as a magnetic core material for various electromagneticvalves, an electronic control fuel injection device and others.

[0003] 2. Description of Related Art

[0004] As the magnetic core for the electromagnetic valve, theelectronic control fuel injection device and the like, the soft magneticferritic stainless steels are practically and frequently used fromdemands to the magnetic properties and the corrosion resistance. Andnow, when the parts made of the soft magnetic ferritic stainless steelare used in a sliding portion or an impacting portion of the apparatusin operation, it often becomes a problem that the deformation throughwearing or buckling causes not only the change of a size but also thedeterioration of magnetic circuit properties and airtightness to bringabout the deterioration of a control accuracy. As a countermeasure forsuch a problem, it is attempted to attain the increase of the strengthand the decrease of the wear by subjecting the sliding portion or theimpacting portion to a surface hardening treatment such as a Cr plating,a nitriding treatment or the like.

[0005] However, such a countermeasure is not preferable because itbrings about the deterioration of the magnetic properties as animportant characteristic for the soft magnetic material and the increaseof the production cost of the apparatus.

[0006] Recently, it is frequently demanded to abolish a step of handlingharmful substances such as chromium and the like, for example, a platingstep in view of the environmental problems, so that it is desired todevelop soft magnetic materials having a good mechanical durability,particularly a high hardness (not less than 270 HV) while ensuringexcellent magnetic properties without passing through the above step.

[0007] As an attempt for increasing the hardness of the ferriticstainless steel itself without deteriorating the magnetic properties,there are found many cases that the hardness is somewhat increased bysolid-solution strengthening with an alloying element (about 170-220HV), but it is not known to develop a highly soft magnetic materialhaving a remarkably high hardness of not less than 270 HV up to thepresent time.

SUMMARY OF THE INVENTION

[0008] It is, therefore, an object of the invention to provide aprecipitation-hardened soft magnetic ferritic stainless steel having notonly excellent magnetic properties and corrosion resistance required asa magnetic core material for various electromagnetic valves, electroniccontrol fuel injection devices and others but also a good coldworkability and a high hardness contributing to the improvement of thedurability such as the wear resistance, the resistance to buckling andthe like.

[0009] Although various precipitation-hardened stainless steels asrepresented by 17-4 PH steel are known up to now, almost all of them area single system consisting of a martensite single phase or an austenitesingle phase as a base of the steel structure, or a dual-phase systemincluding a ferrite phase. There are extremely few cases of studying anddeveloping a precipitation-hardened soft magnetic ferritic stainlesssteel of a single system consisting of a ferrite single phase.Particularly, there is not found a case of developing the ferriticstainless steel having excellent soft magnetic properties except thedescriptions of Japanese Patents 1194892 and 1832191.

[0010] The above Japanese Patent 1194892 is an epoch-making inventiongetting a head start on the precipitation-hardened soft magneticferritic stainless steel and discloses conditions for stabilizing theferrite phase in detail. And also, the above Japanese Patent 1832191 isan invention further improving an appearance luster of the material.

[0011] In both the patents, the hardness after the solution treatmentand the aging treatment has a very high value of not less than 400 HV asa single phase material of the soft magnetic ferrite, but there areproblems that the magnetic properties do not reach to the level requiredwhen using in recent electronic control apparatuses and further the coldworkability is deteriorated due to the excessively high hardness tocause troubles in the mass productivity and the like.

[0012] To solve the above problems, the inventors have made examinationswith respect to a chemical composition in the steel. As a result, theinventors have obtained an extremely effective knowledge for theachievement of the object that the aging precipitation of intermetalliccompounds of Ni and Al added together to the steel (a part of Ti and Zradded is considered to be also included in the compound) is a mainhardening factor of the precipitation-hardened soft magnetic ferriticstainless steel, and hence the hardness after the aging treatment can berendered into a sufficiently practical value of about 300-400 HV withoutdeteriorating the cold workability before the aging treatment and thesoft magnetic properties are more superior to those disclosed in theabove patents.

[0013] The invention is based on the above knowledge and the gistthereof is as follows.

[0014] (1) A precipitation-hardened soft magnetic ferritic stainlesssteel characterized by comprising C: not more than 0.2 mass %, Si:0.01-3.0 mass %, Mn: not more than 0.5 mass %, S: not more than 0.3 mass%, Cr: 12.0-19.0 mass %, Ni: 1.0-4.0 mass % and Al: 0.2-4.0 mass % andfurther containing at least one of Ti: less than 0.5 mass % and Zr: lessthan 0.3 mass %, and the remainder being inevitable impurities and Fe,and having substantially a microstructure of a ferrite phase after asolution treatment and an aging treatment (first invention).

[0015] (2) A precipitation-hardened soft magnetic ferritic stainlesssteel characterized by comprising C: not more than 0.2 mass %, Si:0.01-3.0 mass %, Mn: not more than 0.5 mass %, S: not more than 0.3 mass%, Cr: 12.0-19.0 mass %, Ni: 1.0-4.0 mass % and Al: 0.2-4.0 mass % andfurther containing at least one of Ti: less than 0.5 mass % and Zr: lessthan 0.3 mass % and at least one of Nb: not more than 1.0 mass %, Mo:not more than 4.0 mass %, Cu: not more than 2.0 mass %, B: not more than0.01 mass % and REM: not more than 0.1 mass %, and the remainder beinginevitable impurities and Fe, and having substantially a microstructureof a ferrite phase after a solution treatment and an aging treatment(second invention).

DETAILED DESCRIPTION OF THE INVENTION

[0016] In the invention, the reason of limiting the chemical compositionin the steel to the above range will be explained below. Moreover, mass% in the chemical composition of the steel represents simply as “%”.

[0017] C: not more than 0.2%

[0018] C is an austenite stabilizing element obstructing the formationof steel microstructure based on the ferrite phase and also adverselyaffecting the magnetic properties. To this end, the C content isdesirable to be decreased as far as possible and is limited to not morethan 0.2% in view of the fixation as a carbide or a carbosulfide by Ti,Zr and Nb and the productivity.

[0019] Si: 0.01-3.0%

[0020] Si is not only a useful element as a deoxidizing agent in thestainless steel but also a ferrite stabilizing element, and effectivelycontributes to the increase of the maximum magnetic permeability and thedecrease of the coercive force among the magnetic properties, and isalso a useful element for increasing the specific resistance to improvethe response in a high-frequency zone, and is large in the effect ofincreasing the hardness of the ferrite phase. In order to develop theeffect, Si content is required to be not less than 0.01%. However, whenthe Si content exceeds 3.0%, the cold workability is obstructed tobrings about the lowering of the productivity. Therefore, the Si contentis limited to 0.01-3.0%.

[0021] Mn: not more than 0.5%

[0022] Mn is an useful element as a deoxidizing agent in the stainlesssteel and has effects of fixing S as a sulfide and further improving themachinability. However, since Mn is an austenite stabilizing element,when Mn is excessively added in an amount exceeding 0.5%, the ferritephase is destabilized and further the magnetic properties and thecorrosion resistance are deteriorated, so that the Mn content is limitedto not more than 0.5%. Moreover, the lower limit of the Mn content isnot especially limited, but is preferable to be 0.05% in order toremarkably develop the above effects.

[0023] S: not more than 0.3%

[0024] Since S tends to deteriorate the magnetic properties likewise C,it is desirable to decrease S content as far as possible. Considering apoint that S can prevent the deterioration of the magnetic properties toa some extent through the fixation effect with Mn, Ti and Zr, the Scontent is limited to not more than 0.3%. Moreover, since S is also anelement for improving the machinability, in case of the stainless steelrequiring the machinability, the S content is preferable to be not lessthan 0.02%.

[0025] Cr: 12.0-19.0%

[0026] Cr is one of major components in the ferritic stainless steelaccording to the invention and is an element stabilizing the ferritephase and also effectively improving the corrosion resistance andincreasing the specific resistance. However, when Cr content is lessthan 12.0%, these effects are poor, while when Cr is added in an amountexceeding 19.0%, the magnetic properties are adversely affected.Therefore, the Cr content is limited to 12.0-19.0%.

[0027] Ni: 1.0-4.0%

[0028] Ni is an element having an effect that it is precipitated insteel as an intermetallic compound together with Al after the solutiontreatment and the aging treatment to increasing the hardness. In orderto develop such an effect, Ni content is required to be not less than1.0%. However, the excess addition of Ni is apt to easily induce theformation of the martensite phase or the austenite phase during thesolution treatment, the upper limit of the Ni content is 4.0% as a limitforming substantially the ferrite single-phase in view of the ferritestabilizing effect of the other adding elements.

[0029] Al: 0.2-4.0%

[0030] Al is precipitated in the steel as an intermetallic compoundtogether with Ni to increase the hardness and is an element useful as adeoxidizing agent and also has a ferrite stabilizing action. Moreover,after the solution treatment and the aging treatment, Al added in anamount larger than the amount forming the intermetallic compound with Nihas actions for increasing the maximum magnetic permeability, andlowering the coercive force, and further contributing to the increase ofthe specific resistance to improve the response in a high-frequency zonelikewise Si. To this end, Al content is limited to not less than 0.2% inview of the Ni content. However, the excess addition of Al exceeding4.0% not only needs a special refining process but also obstructs thecold workability, the upper limit of Al content is 4.0%.

[0031] At least one of Ti: less than 0.5% and Zr: less than 0.3% SinceTi and Zr are elements effective for. fixing C and S to enhance themagnetic properties and the corrosion resistance, it is required tocontain at least one of Ti and Zr in the invention. Also, it isconsidered that a part of Ti and Zr contributes to the increase of thehardness by solid-soluting into the intermetallic compounds of Ni and Alafter the solution treatment and the aging treatment. The Ti content ofnot less than 0.5% and/or the Zr content of not less than 0.3%contribute to the increase of the hardness, but obstruct the coldworkability to lower the productivity, so that the Ti content is limitedto less than 0.5% and the Zr content is limited to less than 0.3%.Moreover, in order to develop the effect of enhancing the magneticproperties and the corrosion resistance, it is preferable that the Ticontent is not less than 0.1% and the Zr content is not less than 0.01%.

[0032] In the invention, at least one of Nb: not more than 1.0%, Mo: notmore than 4.0%, Cu: not more than 2.0%, B: not more than 0.01% and REM:not more than 0.1% can be contained, if necessary, in addition to theabove components.

[0033] Nb: not more than 1.0%

[0034] Nb is an element effective for fixing C to enhance the magneticproperties and the corrosion resistance. However, the excess addition ofNb exceeding 1.0% rather obstructs the magnetic properties and the coldworkability, so that the Nb content is limited to not more than 1.0%.

[0035] Mo: not more than 4.0%

[0036] Mo is a ferrite stabilizing element and is an element effectivefor improving the corrosion resistance. However, the excess addition ofMo exceeding 4.0% obstructs the cold workability and lowers theproductivity, so that the Mo content is limited to not more than 4.0%.

[0037] Cu: not more than 2.0%

[0038] Cu is an element effective for improving the corrosion resistanceand also contributes to the age hardening. However, the excess additionof Cu exceeding 2.0% brings about the embrittlement, and complicates thecold working such as cold drawing, straightening or the like, and lowersthe productivity, so that the Cu content is limited to not more than2.0%.

[0039] B: not more than 0.01%, REM: not more than 0.1%

[0040] Both B and REM contribute to improve the cold workability, butwhen B and REM exceed 0.01% and 0.1%, respectively, they rather become afactor obstructing the cold workability. Therefore, the B content andREM content are limited to not more than 0.01% and not more than 0.1%,respectively.

[0041] An embodiment of the process for producing theprecipitation-hardened soft magnetic ferritic stainless steel accordingto the invention will be explained below.

[0042] At first, a raw material of steel having the above chemicalcomposition is melted and refined in, for example, a vacuum inductionfurnace, and shaped into an ingot, which is bloomed at 1000-1100° C. andheated to 1000-1100° C. and hot rolled to form a member having a shapeof wire, rod or plate.

[0043] Then, the member is annealed at 750-1100° C. and subjected tocold drawing and straightening at a reduction of area of 5-25% in caseof the wire member or to a cold straightening in case of the rod orplate member.

[0044] Thereafter, the precipitation-hardened soft magnetic ferriticstainless steel according to the invention can be manufactured bysubjecting to a solution treatment wherein the member is heated to 1000-1100° C., held at this temperature for 1-2 hours and quenched with aforced air cooling fan, water spray or the like, and subjecting to astraightening, and subjecting to an age hardening treatment wherein themember is heated and held at a temperature of 500-600° C. for 2-3 hoursand air-cooled or slowly cooled with nitrogen gas or the like.

[0045] Moreover, the age hardening treatment may be performed after theparts are worked at a state subjected to the solution treatment and thestraightening. Alternatively, after the parts are worked at a statesubjected to the cold straightening, the solution treatment of heatingunder vacuum or in an atmosphere furnace and quenching with nitrogen gasor the like, and the age hardening treatment may be performed.

[0046] The following examples are given in illustration of the inventionand are not intended as limitations thereof.

[0047] Each of various steel raw materials having a chemical compositionshown in Table 1 and a weight of 7 kg is melted in an Ar gas stream andpoured into a mold to form an ingot of 80 mm in diameter. Next, theingot is subjected to a hot forging at 1000-1050° C. into a rod of 24 mmin diameter, which is further subjected to a hot swaging at 1000-1050°C. into a rod of 18 mm in diameter and annealed at 900° C. and thensubjected to a cold drawing to form a rod of 17 mm in diameter for usein various tests. With respect to the thus obtained rods, the hardness,magnetic properties, corrosion resistance and cold workability areexamined to obtain results as shown in Table 2. Moreover, the magneticproperties are measured by using a B-H loop tracer after a ling specimenhaving an outer diameter of 10 mm, an inner diameter of 4.5 mm and athickness of 5 mm is prepared from the rod and subjected to a solutiontreatment by heating in a vacuum furnace at 1050° C. for 1 hour andquenching with nitrogen gas and subsequently to an aging treatment at550° C. for 2 hours. Further, the hardness is measured by using the samespecimen. The corrosion resistance is evaluated by observing a degree ofrust formed on a surface of a specimen after an aqueous solution of 5%NaCi is sprayed at 35° C. for 48 hours onto the specimen made bysubjecting a rod having a diameter of 15 mm and a length of 100 mm tothe same heat treatment as in the specimen for the evaluation of themagnetic properties and polishing with 800 emery paper. Moreover, theevaluation of the corrosion resistance is conducted by two stages of “O”representing a case that the rust is not formed or it is locally andthinly formed on a comer portion of a tip of the rod and “X”representing a case that the rust formation is clear observed. And also,the cold workability is evaluated by presence or absence of theoccurrence of defects such as pull crack, breakage and the like in thecold drawing for the preparation of the rod to be tested, and moreconcretely evaluated by two stages of “O” representing a case that thecold drawing can easily be performed without causing the defects and “X”representing a case that the defects are caused and it is difficult toconduct a mass production process including the cold working. TABLE 1Sample No. C Si Mn P S Cr Ni Al Ti Zr other components Example 1 0.0120.97 0.19 0.006 0.012 14.27 2.74 0.98 0.17 — 2 0.131 2.34 0.14 0.0050.210 18.24 2.76 0.91 0.08 0.180 3 0.005 0.03 0.38 0.006 0.008 12.403.44 3.15 0.38 — 4 0.011 0.99 0.20 0.007 0.013 14.21 2.72 0.97 0.18 —Mo: 2.8 Cu: 1.76 B:0.001 5 0.150 2.64 0.15 0.006 0.230 18.34 2.78 0.880.46 0.110 Mo: 0.08 B: 0.03 6 0.007 0.02 0.36 0.006 0.004 12.40 3.553.20 0.06 — Nb: 0.72 REM: 0.06 7 0.014 0.98 0.21 0.005 0.014 14.65 2.810.96 — 0.080 Comparative 8 0.015 0.84 0.23 0.005 0.13 10.20 3.34 0.870.12 — Mo: 1.18 Cu: 0.34 Example 9 0.013 0.98 0.17 0.006 0.017 16.235.11 1.18 0.16 — No: 1.86 10 0.008 1.56 0.19 0.006 0.018 14.70 0.47 0.160.14 — No: 1.13 11 0.011 0.95 0.18 0.005 0.012 14.30 2.73 0.99 — — 120.012 1.01 0.18 0.004 0.014 13.80 2.71 0.98 0.19 — Cu: 2.60

[0048] TABLE 2 During cold After solution treatment After solutiontreatment and aging treatment working Coercive Coercive Sample ColdHardness Magnetic flux density (T) force Hardness Magnetic flux density(T) force Corrosion No. workability (HV5) B₂ B₅ B₂₅ (A/cm) (HV5) B₂ B₅B₂₅ (A/cm) resistance Example 1 ∘ 203 0.76 1.02 1.27 0.59 343 0.73 0.991.26 0.61 ◯ 2 ◯ 238 0.49 0.93 1.19 1.05 371 0.47 0.92 1.19 1.07 ◯ 3 ◯221 0.59 1.01 1.23 0.83 384 0.61 1.02 1.23 0.81 ◯ 4 ◯ 234 0.43 0.86 1.190.98 363 0.42 0.84 1.18 1.00 ◯ 5 ◯ 241 0.48 0.93 1.19 1.06 375 0.47 0.931.19 1.08 ◯ 6 ◯ 243 0.38 0.99 1.22 1.03 391 0.38 0.81 1.20 1.05 ◯ 7 ◯211 0.74 1.03 1.27 0.63 348 0.71 1.01 1.27 0.65 ◯ Compar- 8 ◯ 254 0.140.42 0.73 3.13 307 0.12 0.38 0.71 3.45 X ative 9 X 338 0.04 0.14 0.614.24 411 0.03 0.15 0.65 4.27 ◯ Example 10 ◯ 205 0.79 1.06 1.26 0.56 2410.78 1.04 1.25 0.55 ◯ 11 ◯ 234 0.09 0.34 0.88 2.28 342 0.07 0.33 0.872.32 ◯ 12 X 238 0.64 0.93 1.22 0.78 353 0.55 0.90 1.22 0.86 ◯

[0049] As seen from the results shown in Table 2, all of Sample Nos. 1-7of the examples are excellent in the corrosion resistance and the coldworkability, and have a hardness of not less than 340 HV5 after theaging treatment, and the magnetic properties have substantially nodifference before and after the aging treatment and show excellentvalues as a soft magnetic material.

[0050] On the other hand, Sample No. 8 of the comparative example is lowin the Cr content and Sample No. 9 of the comparative example is toolarge in the Ni content, so that they are largely influenced by theaustenite stabilizing element and the value of the magnetic flux densityis less than 1 T at B₂₅ and the coercive force is high, which are poorin the magnetic properties as a soft magnetic material. Sample No. 10 ofthe comparative example is excellent in the magnetic properties but thehardness even after the aging treatment is far below 300 HV5 because theamounts of Ni and Al added are too small. Sample No. 11 of thecomparative example is poor in the magnetic properties because theelements such as Ti, Zr and the like for strongly fixing C and S are notadded into steel. Sample No 12 of the comparative example is excellentin the hardness and the magnetic properties, but is poor in the coldworkability because the amount of Cu added is too high, and the breakageand the pull crack frequently occur in the cold drawing for thepreparation of the test steel, and the yield in the preparation of thetest steel is very low.

[0051] According to the invention, there can be providedprecipitation-hardened soft magnetic ferritic stainless steels havingnot only excellent magnetic properties and corrosion resistance but alsoa good cold workability and a high hardness contributing to improve thedurability such as wear resistance, resistance to buckling and the like.When such stainless steels are used as a magnetic core material forvarious electromagnetic valves, electronic control fuel injectiondevices and the like, the improvement of the durability and thereduction of the production cost are attained and the environmentalprotection is good, so that the invention greatly contributes to theindustrial circles.

What is claimed is:
 1. A precipitation-hardened soft magnetic feiriticstainless steel characterized by comprising C: not more than 0.2 mass %,Si: 0.01-3.0 mass %, Mn: not more than 0.5 mass %, S: not more than 0.3mass %, Cr: 12.0-19.0 mass %, Ni: 1.0-4.0 mass % and Al: 0.2-4.0 mass %and further containing at least one of Ti: less than 0.5 mass % and Zr:less than 0.3 mass %, and the remainder being inevitable impurities andFe, and having substantially a microstructure of a ferrite phase after asolution treatment and an aging treatment.
 2. A precipitation-hardenedsoft magnetic ferritic stainless steel characterized by comprising C:not more than 0.2 mass %, Si: 0.01-3.0 mass %, Mn: not more than 0.5mass %, S: not more than 0.3 mass %, Cr: 12.0-19.0 mass %, Ni: 1.0-4.0mass % and Al: 0.2-4.0 mass % and further containing at least one of Ti:less than 0.5 mass % and Zr: less than 0.3 mass % and at least one ofNb: not more than 1.0 mass %, Mo: not more than 4.0 mass %, Cu: not morethan 2.0 mass %, B: not more than 0.01 mass % and REM: not more than 0.1mass %, and the remainder being inevitable impurities and Fe, and havingsubstantially a microstructure of a ferrite phase after a solutiontreatment and an aging treatment.